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Financial Highlights (Per Unit Performance Data and Other Supplemental Financial Data) (Details) (USD $)
 3 Months Ended
Mar. 31, 2015
Mar. 31, 2014
Dec. 06, 2007
Per Share Operating Performance:      
Net asset value, beginning of period $ 26.59usl_NetAssetValuePerUnit $ 42.83usl_NetAssetValuePerUnit $ 50.00usl_NetAssetValuePerUnit
Total income (loss) $ (2.58)usl_TotalIncomeLossPerUnit $ 1.38usl_TotalIncomeLossPerUnit  
Total expenses $ (0.05)usl_TotalExpensesPerUnit $ (0.11)usl_TotalExpensesPerUnit  
Net increase (decrease) in net asset value $ (2.63)usl_IncreaseDecreaseInNetAssetValuePerUnit $ 1.27usl_IncreaseDecreaseInNetAssetValuePerUnit  
Net asset value, end of period $ 23.96usl_NetAssetValuePerUnit $ 44.10usl_NetAssetValuePerUnit $ 50.00usl_NetAssetValuePerUnit
Total Return (9.89%)usl_RateOfReturnOnAverageAsset 2.97%usl_RateOfReturnOnAverageAsset  
Ratios to Average Net Assets      
Total income (loss) (8.12%)usl_PercentageOfTotalRevenue 2.80%usl_PercentageOfTotalRevenue  
Expenses excluding management fees 0.21%usl_ExpensesExcludingManagementFeePercentage [1] 0.41%usl_ExpensesExcludingManagementFeePercentage [1]  
Management fees 0.60%usl_ManagementIncentiveFeePercentage [1] 0.60%usl_ManagementIncentiveFeePercentage [1]  
Net income (loss) (8.32%)usl_PercentageOfConsolidatedNetIncomeLoss 2.55%usl_PercentageOfConsolidatedNetIncomeLoss  
[1] Annualized